1. Field of the Invention
This invention relates to a mixing device and method for effectively stirring or mixing different materials such as raw materials for the manufacture of a variety of concretes.
2. Description of the Prior Art
Mixing devices such as a concrete mixer for mixing cement, water and aggregate to produce cement paste or ready-mixed concrete are commonly divided into three types. One is a vessel-rotating type mixer (so-called vessel-tilting type mixer) comprising a rotary vessel and inner mixing blades fixed inside the mixing vessel. In this vessel-rotating type mixer, the rotary vessel is rotated to mix different materials by use of free fall of the materials in the rotating vessel. Thus, this mixer making use of gravitation is inferior in efficiency.
A second type is a so-called pan-type mixer having a stationary pan-like vessel and a mixing paddle disposed on the axial center of the vessel.
A third type is a horizontal-paddle type mixer having a stationary vessel and one or more rotary mixing paddles horizontally supported in the vessel.
The aforementioned vessel-fixed type mixers are now finding widespread acceptance for actual use. In the concrete mixer of this type, however, mixing materials are mixed with a shearing force produced by rotating the paddles, and therefore, cannot be satisfactorily circulated by convection in the vessel and well mixed in a short time.
There has been a conventional concrete mixer of the vessel-fixed type for mixing powder material such as flour and granular medicines, as shown in FIG. 1. This prior art concrete mixer has a single agitating spiral screw 2 vertically supported in a vessel 4, and a draft cylinder 6 arranged coaxially around the screw 2 so as to circulate the mixing material in the vessel by convection. That is, by rotating the screw 2, the mixing material M in the vessel 4 is caused to move downward inside the draft cylinder 6 and upward outside the draft cylinder 6.
In the case of dealing with sticky mixing materials in the concrete mixer using such a draft cylinder around the screw, however, the mixing material M near the screw 2 and at the inner surface of the vessel 4 tends to be prevented from moving and is stagnated around the regions as indicated by the symbols T1 and T2 in FIG. 1. Consequently, the mixture obtained is insufficiently mixed.
Furthermore, the situation that the sticky mixing material within the draft cylinder 6 is agglutinated to the screw 2 while being mixed becomes a matter of great concern. As a result, the screw 2 is rotated in sympathy with the mixing material, thus bringing about a so-called "racing" phenomenon in which the mixing material in the vessel is not circulated by convection, thus causing the mixer to malfunction practically. The racing phenomenon conspicuously occurs when mixing material having high viscosity.
Also in a dual-screw type mixer having two screws arranged in parallel within a mixing vessel, mixing material admitted into the vessel is apt to lose fluidity around the inner surface of the vessel and to be stagnant. Under certain circumstances, the mixing material is possibly agglutinated to the screws, consequently bringing about the racing phenomenon causing the mixer to malfunction practically.
Thus, conventional concrete mixers of all types have a common disadvantage such that they cannot uniformly stir or mix different mixing materials with a high efficiency and are apt to give rise to the racing phenomenon in which the mixing materials are agglutinated to and rotated together with the rotating screw or screws.
Considering the case of cement paste for light-weight concrete or aerated concrete by way of example, the insufficient mixing as noted above entails a problem of bringing forth small cement bubbles in the cement paste. The cement bubbles in the cement paste result in defects on the micron order in hardened concrete, thus lowering the strength of the concrete. Although the mixing materials should be sufficiently mixed to obviate such problems and obtain concrete products of high quality, it has been desired to enhance the efficiency of production of the cement paste so as to produce cement paste in a short period of time in large quantities in view of productivity.